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RFC2315 PKCS#7: Cryptographic Message Syntax

Definitions

PEM: Internet Privacy-Enhanced Mail, as defined in RFCs 1421-1424

BER: Basic Encoding Rules for ASN.1, as defined in X.690

DER: Distinguished Encoding Rules for ASN.1, as defined in X.690. DER is a subset of BER.

General syntax

ContentInfo

ContentInfo ::= SEQUENCE {
     contentType ContentType,
     content
       [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL }

ContentType: data, signedData, envelopedData, signedAndEnvelopedData, digestedData, and encryptedData

对于 signed-data, signed-and-enveloped-data, or digested-data 的数据，会先对DER编码的content作hash计算

对于 enveloped-data or signed-and-enveloped-data 的数据，会根据content-encryption algorithm对内容做加密，写入定长的BER编码content域

SignedData

SignedData ::= SEQUENCE {
     version Version,
     digestAlgorithms DigestAlgorithmIdentifiers,
     contentInfo ContentInfo,
     certificates
        [0] IMPLICIT ExtendedCertificatesAndCertificates
          OPTIONAL,
     crls
       [1] IMPLICIT CertificateRevocationLists OPTIONAL,
     signerInfos SignerInfos }

SignerInfo

SignerInfo ::= SEQUENCE {
     version Version,
     issuerAndSerialNumber IssuerAndSerialNumber,
     digestAlgorithm DigestAlgorithmIdentifier,
     authenticatedAttributes
       [0] IMPLICIT Attributes OPTIONAL,
     digestEncryptionAlgorithm
       DigestEncryptionAlgorithmIdentifier,
     encryptedDigest EncryptedDigest,
     unauthenticatedAttributes
       [1] IMPLICIT Attributes OPTIONAL }

DigestInfo

DigestInfo ::= SEQUENCE {
     digestAlgorithm DigestAlgorithmIdentifier,
     digest Digest }

Enveloped-data content type

EnvelopedData

EnvelopedData ::= SEQUENCE {
     version Version,
     recipientInfos RecipientInfos,
     encryptedContentInfo EncryptedContentInfo }

   RecipientInfos ::= SET OF RecipientInfo

   EncryptedContentInfo ::= SEQUENCE {
     contentType ContentType,
     contentEncryptionAlgorithm
       ContentEncryptionAlgorithmIdentifier,
     encryptedContent
       [0] IMPLICIT EncryptedContent OPTIONAL }

   EncryptedContent ::= OCTET STRING

RecipientInfo

encryptedKey 是用recipient’s public key加密content-encryption key的结果，确保只有recipient能解密获取content-encryption key

RecipientInfo ::= SEQUENCE {
     version Version,
     issuerAndSerialNumber IssuerAndSerialNumber,
     keyEncryptionAlgorithm

       KeyEncryptionAlgorithmIdentifier,
     encryptedKey EncryptedKey }

   EncryptedKey ::= OCTET STRING

PKCS#7 PADDING

假设内容长度为l，要求k字节对齐，则：

   01 -- if l mod k = k-1
   02 02 -- if l mod k = k-2
   .
   .
   .
   k k ... k k -- if l mod k = 0

Signed-and-enveloped-data content type

SignedAndEnvelopedData

SignedAndEnvelopedData ::= SEQUENCE {
     version Version,
     recipientInfos RecipientInfos,
     digestAlgorithms DigestAlgorithmIdentifiers,
     encryptedContentInfo EncryptedContentInfo,
     certificates
        [0] IMPLICIT ExtendedCertificatesAndCertificates
          OPTIONAL,
     crls
       [1] IMPLICIT CertificateRevocationLists OPTIONAL,
     signerInfos SignerInfos }

Digested-data content type

DigestedData

ContentInfo里的content是被计算摘要的content

DigestedData ::= SEQUENCE {
     version Version,
     digestAlgorithm DigestAlgorithmIdentifier,
     contentInfo ContentInfo,
     digest Digest }

   Digest ::= OCTET STRING

Signed-and-enveloped-data content type 过程

随机生成 content-encryption key

用 recipient’s public key 加密 content-encryption key

加密后的content-encryption key和recipient关联信息，打包到RecipientInfo

根据signer指定的hash算法计算content对应的message-digest

message-digest及其关联信息用signer’s private key加密得到res，再用content-encryption key对res做一次加密。注意第二次加密时，res可能要先padding到固定长度

上面二次加密的内容，以及siger的关联信息，打包到SignerInfo

content用content-encryption key加密

前面提到的message-digest算法、RecipientInfo、SignerInfo、encrypted content一起打包到SignedAndEnvelopedData

当recipient收到数据后，先用自身私钥解密获得content-encryption key；然后解密content；然后解密上面二次加密的message-digest；根据解密的content计算message-digest，与解密得到的message-digest比对。

Published

28 July 2019

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