Erlang -- Getting Started

# 3 Getting Started

This section describes examples of how to use the Public Key API. Keys and certificates used in the following sections are generated only for testing the Public Key application.

Some shell printouts in the following examples are abbreviated for increased readability.

###
3.1 PEM Files

Public-key data (keys, certificates, and so on) can be stored in Privacy Enhanced Mail (PEM) format. The PEM files have the following structure:

        <text>
        -----BEGIN <SOMETHING>-----
        <Attribute> : <Value>
        <Base64 encoded DER data>
        -----END <SOMETHING>-----
        <text>

A file can contain several BEGIN/END blocks. Text lines between blocks are ignored. Attributes, if present, are ignored except for Proc-Type and DEK-Info, which are used when DER data is encrypted.

####
DSA Private Key

A DSA private key can look as follows:

Note

File handling is not done by the Public Key application.

    1> {ok, PemBin} = file:read_file("dsa.pem").
    {ok,<<"-----BEGIN DSA PRIVATE KEY-----\nMIIBuw"...>>}

The following PEM file has only one entry, a private DSA key:

    2> [DSAEntry] =  public_key:pem_decode(PemBin).
    [{'DSAPrivateKey',<<48,130,1,187,2,1,0,2,129,129,0,183,
                        179,230,217,37,99,144,157,21,228,204,
    		    162,207,61,246,...>>,
    		    not_encrypted}]

    3> Key = public_key:pem_entry_decode(DSAEntry).
    #'DSAPrivateKey'{version = 0,
                     p = 12900045185019966618...6593,
                     q = 1216700114794736143432235288305776850295620488937,
                     g = 10442040227452349332...47213,
                     y = 87256807980030509074...403143,
                     x = 510968529856012146351317363807366575075645839654}

####
RSA Private Key with Password

An RSA private key encrypted with a password can look as follows:

    1> {ok, PemBin} = file:read_file("rsa.pem").
    {ok,<<"Bag Attribute"...>>}

The following PEM file has only one entry, a private RSA key:

    2>[RSAEntry] = public_key:pem_decode(PemBin).
    [{'RSAPrivateKey',<<224,108,117,203,152,40,15,77,128,126,
                        221,195,154,249,85,208,202,251,109,
                        119,120,57,29,89,19,9,...>>,
                      {"DES-EDE3-CBC",<<"kÙeø¼pµL">>}}]

In this following example, the password is "abcd1234":

    3> Key = public_key:pem_entry_decode(RSAEntry, "abcd1234").
        #'RSAPrivateKey'{version = 'two-prime',
                     modulus = 1112355156729921663373...2737107,
                     publicExponent = 65537,
                     privateExponent = 58064406231183...2239766033,
                     prime1 = 11034766614656598484098...7326883017,
                     prime2 = 10080459293561036618240...77738643771,
                     exponent1 = 77928819327425934607...22152984217,
                     exponent2 = 36287623121853605733...20588523793,
                     coefficient = 924840412626098444...41820968343,
                     otherPrimeInfos = asn1_NOVALUE}

####
X509 Certificates

The following is an example of X509 certificates:

    1> {ok, PemBin} = file:read_file("cacerts.pem").
    {ok,<<"-----BEGIN CERTIFICATE-----\nMIIC7jCCAl"...>>}

The following file includes two certificates:

    2> [CertEntry1, CertEntry2] = public_key:pem_decode(PemBin).
    [{'Certificate',<<48,130,2,238,48,130,2,87,160,3,2,1,2,2,
                      9,0,230,145,97,214,191,2,120,150,48,13,
                      ...>>,
                    not_encrypted},
     {'Certificate',<<48,130,3,200,48,130,3,49,160,3,2,1,2,2,1,
                      1,48,13,6,9,42,134,72,134,247,...>>,
                    not_encrypted}]

Certificates can be decoded as usual:

    2> Cert = public_key:pem_entry_decode(CertEntry1).
    #'Certificate'{
        tbsCertificate =
            #'TBSCertificate'{
                version = v3,serialNumber = 16614168075301976214,
                signature =
                    #'AlgorithmIdentifier'{
                        algorithm = {1,2,840,113549,1,1,5},
                        parameters = <<5,0>>},
                issuer =
                    {rdnSequence,
                        [[#'AttributeTypeAndValue'{
                              type = {2,5,4,3},
                              value = <<19,8,101,114,108,97,110,103,67,65>>}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,11},
                              value = <<19,10,69,114,108,97,110,103,32,79,84,80>>}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,10},
                              value = <<19,11,69,114,105,99,115,115,111,110,32,65,66>>}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,7},
                              value = <<19,9,83,116,111,99,107,104,111,108,109>>}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,6},
                              value = <<19,2,83,69>>}],
                         [#'AttributeTypeAndValue'{
                              type = {1,2,840,113549,1,9,1},
                              value = <<22,22,112,101,116,101,114,64,101,114,...>>}]]},
                validity =
                    #'Validity'{
                        notBefore = {utcTime,"080109082929Z"},
                        notAfter = {utcTime,"080208082929Z"}},
                subject =
                    {rdnSequence,
                        [[#'AttributeTypeAndValue'{
                              type = {2,5,4,3},
                              value = <<19,8,101,114,108,97,110,103,67,65>>}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,11},
                              value = <<19,10,69,114,108,97,110,103,32,79,84,80>>}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,10},
                              value = <<19,11,69,114,105,99,115,115,111,110,32,...>>}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,7},
                              value = <<19,9,83,116,111,99,107,104,111,108,...>>}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,6},
                              value = <<19,2,83,69>>}],
                         [#'AttributeTypeAndValue'{
                              type = {1,2,840,113549,1,9,1},
                              value = <<22,22,112,101,116,101,114,64,...>>}]]},
                subjectPublicKeyInfo =
                    #'SubjectPublicKeyInfo'{
                        algorithm =
                            #'AlgorithmIdentifier'{
                                algorithm = {1,2,840,113549,1,1,1},
                                parameters = <<5,0>>},
                        subjectPublicKey =
                            {0,<<48,129,137,2,129,129,0,203,209,187,77,73,231,90,...>>}},
                issuerUniqueID = asn1_NOVALUE,
                subjectUniqueID = asn1_NOVALUE,
                extensions =
                    [#'Extension'{
                         extnID = {2,5,29,19},
                         critical = true,
                         extnValue = [48,3,1,1,255]},
                     #'Extension'{
                         extnID = {2,5,29,15},
                         critical = false,
                         extnValue = [3,2,1,6]},
                     #'Extension'{
                         extnID = {2,5,29,14},
                         critical = false,
                         extnValue = [4,20,27,217,65,152,6,30,142|...]},
                     #'Extension'{
                         extnID = {2,5,29,17},
                         critical = false,
                         extnValue = [48,24,129,22,112,101,116,101|...]}]},
        signatureAlgorithm =
            #'AlgorithmIdentifier'{
                algorithm = {1,2,840,113549,1,1,5},
                parameters = <<5,0>>},
        signature =
        <<163,186,7,163,216,152,63,47,154,234,139,73,154,96,120,
        165,2,52,196,195,109,167,192,...>>}

Parts of certificates can be decoded with public_key:der_decode/2, using the ASN.1 type of that part. However, an application-specific certificate extension requires application-specific ASN.1 decode/encode-functions. In the recent example, the first value of rdnSequence is of ASN.1 type 'X520CommonName'. ({2,5,4,3} = ?id-at-commonName):

    public_key:der_decode('X520CommonName', <<19,8,101,114,108,97,110,103,67,65>>).
    {printableString,"erlangCA"}

However, certificates can also be decoded using pkix_decode_cert/2, which can customize and recursively decode standard parts of a certificate:

    3>{_, DerCert, _} = CertEntry1.

    4> public_key:pkix_decode_cert(DerCert, otp).
    #'OTPCertificate'{
        tbsCertificate =
            #'OTPTBSCertificate'{
                version = v3,serialNumber = 16614168075301976214,
                signature =
                    #'SignatureAlgorithm'{
                        algorithm = {1,2,840,113549,1,1,5},
                        parameters = 'NULL'},
                issuer =
                    {rdnSequence,
                        [[#'AttributeTypeAndValue'{
                              type = {2,5,4,3},
                              value = {printableString,"erlangCA"}}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,11},
                              value = {printableString,"Erlang OTP"}}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,10},
                              value = {printableString,"Ericsson AB"}}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,7},
                              value = {printableString,"Stockholm"}}],
                         [#'AttributeTypeAndValue'{type = {2,5,4,6},value = "SE"}],
                         [#'AttributeTypeAndValue'{
                              type = {1,2,840,113549,1,9,1},
                              value = "peter@erix.ericsson.se"}]]},
                validity =
                    #'Validity'{
                        notBefore = {utcTime,"080109082929Z"},
                        notAfter = {utcTime,"080208082929Z"}},
                subject =
                    {rdnSequence,
                        [[#'AttributeTypeAndValue'{
                              type = {2,5,4,3},
                              value = {printableString,"erlangCA"}}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,11},
                              value = {printableString,"Erlang OTP"}}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,10},
                              value = {printableString,"Ericsson AB"}}],
                         [#'AttributeTypeAndValue'{
                              type = {2,5,4,7},
                              value = {printableString,"Stockholm"}}],
                         [#'AttributeTypeAndValue'{type = {2,5,4,6},value = "SE"}],
                         [#'AttributeTypeAndValue'{
                              type = {1,2,840,113549,1,9,1},
                              value = "peter@erix.ericsson.se"}]]},
                subjectPublicKeyInfo =
                    #'OTPSubjectPublicKeyInfo'{
                        algorithm =
                            #'PublicKeyAlgorithm'{
                                algorithm = {1,2,840,113549,1,1,1},
                                parameters = 'NULL'},
                        subjectPublicKey =
                            #'RSAPublicKey'{
                                modulus =
                                    1431267547247997...37419,
                                publicExponent = 65537}},
                issuerUniqueID = asn1_NOVALUE,
                subjectUniqueID = asn1_NOVALUE,
                extensions =
                    [#'Extension'{
                         extnID = {2,5,29,19},
                         critical = true,
                         extnValue =
                             #'BasicConstraints'{
                                 cA = true,pathLenConstraint = asn1_NOVALUE}},
                     #'Extension'{
                         extnID = {2,5,29,15},
                         critical = false,
                         extnValue = [keyCertSign,cRLSign]},
                     #'Extension'{
                         extnID = {2,5,29,14},
                         critical = false,
                         extnValue = [27,217,65,152,6,30,142,132,245|...]},
                     #'Extension'{
                         extnID = {2,5,29,17},
                         critical = false,
                         extnValue = [{rfc822Name,"peter@erix.ericsson.se"}]}]},
        signatureAlgorithm =
            #'SignatureAlgorithm'{
                algorithm = {1,2,840,113549,1,1,5},
                parameters = 'NULL'},
        signature =
             <<163,186,7,163,216,152,63,47,154,234,139,73,154,96,120,
               165,2,52,196,195,109,167,192,...>>}

This call is equivalent to public_key:pem_entry_decode(CertEntry1):

    5> public_key:pkix_decode_cert(DerCert, plain).
    #'Certificate'{ ...}

####
Encoding Public-Key Data to PEM Format

If you have public-key data and want to create a PEM file this can be done by calling functions public_key:pem_entry_encode/2 and pem_encode/1 and saving the result to a file. For example, assume that you have PubKey = 'RSAPublicKey'{}. Then you can create a PEM-"RSA PUBLIC KEY" file (ASN.1 type 'RSAPublicKey') or a PEM-"PUBLIC KEY" file ('SubjectPublicKeyInfo' ASN.1 type).

The second element of the PEM-entry is the ASN.1 DER encoded key data:

    1> PemEntry = public_key:pem_entry_encode('RSAPublicKey', RSAPubKey).
    {'RSAPublicKey', <<48,72,...>>, not_encrypted}

    2> PemBin = public_key:pem_encode([PemEntry]).
    <<"-----BEGIN RSA PUBLIC KEY-----\nMEgC...>>

    3> file:write_file("rsa_pub_key.pem", PemBin).
    ok

or:

    1> PemEntry = public_key:pem_entry_encode('SubjectPublicKeyInfo', RSAPubKey).
    {'SubjectPublicKeyInfo', <<48,92...>>, not_encrypted}

    2> PemBin = public_key:pem_encode([PemEntry]).
    <<"-----BEGIN PUBLIC KEY-----\nMFw...>>

    3> file:write_file("pub_key.pem", PemBin).
    ok

###
3.2 RSA Public-Key Cryptography

Suppose you have the following private key and a corresponding public key:

- PrivateKey = #'RSAPrivateKey{}' and the plaintext Msg = binary()
- PublicKey = #'RSAPublicKey'{}

Then you can proceed as follows:

Encrypt with the private key:

    RsaEncrypted = public_key:encrypt_private(Msg, PrivateKey),
    Msg = public_key:decrypt_public(RsaEncrypted, PublicKey),

Encrypt with the public key:

    RsaEncrypted = public_key:encrypt_public(Msg, PublicKey),
    Msg = public_key:decrypt_private(RsaEncrypted, PrivateKey),

Note

You normally do only one of the encrypt or decrypt operations, and the peer does the other. This normally used in legacy applications as a primitive digital signature.

###
3.3 Digital Signatures

Suppose you have the following private key and a corresponding public key:

- PrivateKey = #'RSAPrivateKey{}' or #'DSAPrivateKey'{} and the plaintext Msg = binary()
- PublicKey = #'RSAPublicKey'{} or {integer(), #'DssParams'{}}

Then you can proceed as follows:

    Signature = public_key:sign(Msg, sha, PrivateKey),
    true = public_key:verify(Msg, sha, Signature, PublicKey),

Note

You normally do only one of the sign or verify operations, and the peer does the other.

It can be appropriate to calculate the message digest before calling sign or verify, and then use none as second argument:

    Digest = crypto:sha(Msg),
    Signature = public_key:sign(Digest, none, PrivateKey),
    true = public_key:verify(Digest, none, Signature, PublicKey),

###
3.4 Verifying a certificate hostname

####
Background

When a client checks a server certificate there are a number of checks available like checks that the certificate is not revoked, not forged or not out-of-date.

There are however attacks that are not detected by those checks. Suppose a bad guy has succeeded with a DNS infection. Then the client could believe it is connecting to one host but ends up at another but evil one. Though it is evil, it could have a perfectly legal certificate! The certificate has a valid signature, it is not revoked, the certificate chain is not faked and has a trusted root and so on.

To detect that the server is not the intended one, the client must additionally perform a hostname verification. This procedure is described in RFC 6125. The idea is that the certificate lists the hostnames it could be fetched from. This is checked by the certificate issuer when the certificate is signed. So if the certificate is issued by a trusted root the client could trust the host names signed in it.

There is a default hostname matching procedure defined in RFC 6125, section 6 as well as protocol dependent variations defined in RFC 6125 appendix B. The default procedure is implemented in public_key:pkix_verify_hostname/2,3. It is possible for a client to hook in modified rules using the options list.

Some terminology is needed: the certificate presents hostname(s) on which it is valid. Those are called Presented IDs. The hostname(s) the client believes it connects to are called Reference IDs. The matching rules aims to verify that there is at least one of the Reference IDs that matches one of the Presented IDs. If not, the verification fails.

The IDs contains normal fully qualified domain names like e.g foo.example.com, but IP addresses are not recommended. The rfc describes why this is not recommended as well as security considerations about how to acquire the Reference IDs.

Internationalized domain names are not supported.

####
The verification process

Traditionally the Presented IDs were found in the Subject certificate field as CN names. This is still quite common. When printing a certificate they show up as:

     $ openssl x509 -text < cert.pem
     ...
     Subject: C=SE, CN=example.com, CN=*.example.com, O=erlang.org
     ...

The example Subject field has one C, two CN and one O part. It is only the CN (Common Name) that is used by hostname verification. The two other (C and O) is not used here even when they contain a domain name like the O part. The C and O parts are defined elsewhere and meaningful only for other functions.

In the example the Presented IDs are example.com as well as hostnames matching *.example.com. For example foo.example.com and bar.example.com both matches but not foo.bar.example.com. The name erlang.org matches neither since it is not a CN.

In case where the Presented IDs are fetched from the Subject certificate field, the names may contain wildcard characters. The function handles this as defined in chapter 6.4.3 in RFC 6125.

There may only be one wildcard character and that is in the first label, for example: *.example.com. This matches foo.example.com but neither example.com nor foo.bar.example.com.

There may be label characters before or/and after the wildcard. For example: a*d.example.com matches abcd.example.com and ad.example.com, but not ab.cd.example.com.

In the previous example there is no indication of which protocols are expected. So a client has no indication of whether it is a web server, an ldap server or maybe a sip server it is connected to. There are fields in the certificate that can indicate this. To be more exact, the rfc introduces the usage of the X509v3 Subject Alternative Name in the X509v3 extensions field:

     $ openssl x509 -text < cert.pem
     ...
     X509v3 extensions:
         X509v3 Subject Alternative Name:
             DNS:kb.example.org, URI:https://www.example.org
     ...

Here kb.example.org serves any protocol while www.example.org presents a secure web server.

The next example has both Subject and Subject Alternate Name present:

     $ openssl x509 -text < cert.pem
     ...
     Subject: C=SE, CN=example.com, CN=*.example.com, O=erlang.org
     ...
     X509v3 extensions:
         X509v3 Subject Alternative Name:
             DNS:kb.example.org, URI:https://www.example.org
     ...

The RFC states that if a certificate defines Reference IDs in a Subject Alternate Name field, the Subject field MUST NOT be used for host name checking, even if it contains valid CN names. Therefore only kb.example.org and https://www.example.org matches. The match fails both for example.com and foo.example.com because they are in the Subject field which is not checked because the Subject Alternate Name field is present.

####
Function call examples

Note

Other applications like ssl/tls or https might have options that are passed down to the public_key:pkix_verify_hostname. You will probably not have to call it directly

Suppose our client expects to connect to the web server https://www.example.net. This URI is therefore the Reference IDs of the client. The call will be:

     public_key:pkix_verify_hostname(CertFromHost,
                                     [{uri_id, "https://www.example.net"}
                                     ]).

The call will return true or false depending on the check. The caller do not need to handle the matching rules in the rfc. The matching will proceed as:

- If there is a Subject Alternate Name field, the {uri_id,string()} in the function call will be compared to any {uniformResourceIdentifier,string()} in the Certificate field. If the two strings() are equal (case insensitive), there is a match. The same applies for any {dns_id,string()} in the call which is compared with all {dNSName,string()} in the Certificate field.
- If there is NO Subject Alternate Name field, the Subject field will be checked. All CN names will be compared to all hostnames extracted from {uri_id,string()} and from {dns_id,string()}.

####
Extending the search mechanism

The caller can use own extraction and matching rules. This is done with the two options fqdn_fun and match_fun.

##### Hostname extraction

The fqdn_fun extracts hostnames (Fully Qualified Domain Names) from uri_id or other ReferenceIDs that are not pre-defined in the public_key function. Suppose you have some URI with a very special protocol-part: myspecial://example.com". Since this a non-standard URI there will be no hostname extracted for matching CN-names in the Subject.

To "teach" the function how to extract, you can give a fun which replaces the default extraction function. The fqdn_fun takes one argument and returns either a string() to be matched to each CN-name or the atom default which will invoke the default fqdn extraction function. The return value undefined removes the current URI from the fqdn extraction.

     ...
     Extract = fun({uri_id, "myspecial://"++HostName}) -> HostName;
                  (_Else) -> default
               end,
     ...
     public_key:pkix_verify_hostname(CertFromHost, RefIDs,
                                     [{fqdn_fun, Extract}])
     ...

##### Re-defining the match operation

The default matching handles dns_id and uri_id. In an uri_id the value is tested for equality with a value from the Subject Alternate Name. If some other kind of matching is needed, use the match_fun option.

The match_fun takes two arguments and returns either true, false or default. The value default will invoke the default match function.

     ...
     Match = fun({uri_id,"myspecial://"++A},
                 {uniformResourceIdentifier,"myspecial://"++B}) ->
                                                        my_match(A,B);
                (_RefID, _PresentedID) ->
                                    default
             end,
     ...
     public_key:pkix_verify_hostname(CertFromHost, RefIDs,
                                     [{match_fun, Match}]),
     ...

In case of a match operation between a ReferenceID and a CN value from the Subject field, the first argument to the fun is the extracted hostname from the ReferenceID, and the second argument is the tuple {cn, string()} taken from the Subject field. That makes it possible to have separate matching rules for Presented IDs from the Subject field and from the Subject Alternate Name field.

The default matching transformes the ascii values in strings to lowercase before comparing. The match_fun is however called without any transformation applied to the strings. The reason is to enable the user to do unforeseen handling of the strings where the original format is needed.

####
"Pinning" a Certificate

The RFC 6125 defines pinning as:

"The act of establishing a cached name association between the application service's certificate and one of the client's reference identifiers, despite the fact that none of the presented identifiers matches the given reference identifier. ..."

The purpose is to have a mechanism for a human to accept an otherwise faulty Certificate. In for example a web browser, you could get a question like

Warning: you wanted to visit the site www.example.com, but the certificate is for shop.example.com. Accept anyway (yes/no)?"

This could be accomplished with the option fail_callback which will be called if the hostname verification fails:

     -include_lib("public_key/include/public_key.hrl"). % Record def
     ...
     Fail = fun(#'OTPCertificate'{}=C) ->
                  case in_my_cache(C) orelse my_accept(C) of
                      true ->
                           enter_my_cache(C),
                           true;
                      false ->
                           false
             end,
     ...
     public_key:pkix_verify_hostname(CertFromHost, RefIDs,
                                     [{fail_callback, Fail}]),
     ...