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diff --git a/src/x509.erl b/src/x509.erl
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+++ b/src/x509.erl
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+%%% Copyright (c) 2014, NORDUnet A/S.
+%%% See LICENSE for licensing information.
+
+-module(x509).
+-export([valid_chain_p/3, detox_precert/1]).
+-include_lib("public_key/include/public_key.hrl").
+-type der_encoded() :: public_key:der_encoded().
+
+%%%%%%%%%%%%%%%%%%%%
+%% @doc Verify that the leaf cert or precert has a valid chain back to
+%% an acceptable root cert. Order of certificates in second argument
+%% is: leaf cert in head, chain in tail. Order of first argument is
+%% irrelevant.
+
+-spec valid_chain_p([der_encoded()], [der_encoded()], integer()) -> boolean().
+valid_chain_p(_, _, MaxChainLength) when MaxChainLength =< 0 ->
+    false;
+valid_chain_p(AcceptableRootCerts, [TopCert], MaxChainLength) ->
+    case lists:member(TopCert, AcceptableRootCerts) of
+        true ->                           % Top cert is part of chain.
+            true;
+        false ->     % Top cert might be signed by cert in truststore.
+            (MaxChainLength > 1) and
+                lists:any(fun(X) ->
+                                  signed_by_p(TopCert, X) end,
+                          AcceptableRootCerts)
+    end;
+valid_chain_p(AcceptableRootCerts, [BottomCert|Rest], MaxChainLength) ->
+    case signed_by_p(BottomCert, hd(Rest)) of
+        false -> false;
+        true -> valid_chain_p(AcceptableRootCerts, Rest, MaxChainLength - 1)
+    end.
+
+-spec signed_by_p(der_encoded(), der_encoded()) -> boolean().
+signed_by_p(Cert, IssuerCert) ->
+    %% FIXME: Validate presence and contents (against constraints) of
+    %% names (subject, subjectAltName, emailAddress) too?
+    case public_key:pkix_is_issuer(Cert, IssuerCert) of
+        true ->           % Cert.issuer does match IssuerCert.subject.
+            public_key:pkix_verify(Cert, public_key(IssuerCert));
+        false ->
+            false
+    end.
+
+-spec public_key(der_encoded() | #'OTPCertificate'{}) -> public_key:public_key().
+public_key(CertDer) when is_binary(CertDer) ->
+    public_key(public_key:pkix_decode_cert(CertDer, otp));
+public_key(#'OTPCertificate'{
+              tbsCertificate =
+                  #'OTPTBSCertificate'{subjectPublicKeyInfo =
+                                           #'OTPSubjectPublicKeyInfo'{
+                                              subjectPublicKey = Key}}}) ->
+    Key.
+
+%%%%%%%%%%%%%%%%%%%%
+%% Precertificates according to draft-ietf-trans-rfc6962-bis-04.
+
+%% Submitted precerts have a special critical poison extension -- OID
+%% 1.3.6.1.4.1.11129.2.4.3, whose extnValue OCTET STRING contains
+%% ASN.1 NULL data (0x05 0x00).
+
+%% They are signed with either the CA cert that will sign the final
+%% cert or Precertificate Signing Certificate directly signed by the
+%% CA cert that will sign the final cert. A Precertificate Signing
+%% Certificate has CA:true and Extended Key Usage: Certificate
+%% Transparency, OID 1.3.6.1.4.1.11129.2.4.4.
+
+%% A PreCert in a SignedCertificateTimestamp does _not_ contain the
+%% poison extension, nor a Precertificate Signing Certificate. This
+%% means that we might have to 1) remove poison extensions in leaf
+%% certs, 2) remove "poisoned signatures", 3) change issuer and
+%% Authority Key Identifier of leaf certs.
+
+-spec detox_precert([#'Certificate'{}]) -> [#'Certificate'{}].
+detox_precert(CertChain) ->
+    CertChain.                                  % NYI