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|  |  | To access the contents, click the chapter and section titles. Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth) (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book:
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PEM Documents The specifications for PEM come from four documents: — RFC 1421: Part I, Message Encryption and Authentication Procedures. This document defines message encryption and authentication procedures in order to provide privacy-enhanced mail services for electronic mail transfer on the Internet. — RFC 1422: Part II, Certificate-Based Key Management. This document defines a supporting key management architecture and infrastructure, based on public-key certificate techniques to provide keying information to message originators and recipients. — RFC 1423: Part III, Algorithms, Modes, and Identifiers. This document provides definitions, formats, references, and citations for cryptographic algorithms, usage modes, and associated identifiers and parameters. — RFC 1424: Part IV, Key Certification and Related Services. This document describes three types of service in support of PEM: key certification, certificate revocation list (CRL) storage, and CRL retrieval. Certificates PEM is compatible with the authentication framework described in [304]; see also [826]. PEM is a superset of X.509; it establishes procedures and conventions for a key-management infrastructure for use with PEM and with other protocols (from both the TCP/IP and OSI suites) in the future. The key-management infrastructure establishes a single root for all Internet certification. The Internet Policy Registration Authority (IPRA) establishes global policies that apply to all certification under this hierarchy. Beneath the IPRA root are Policy Certification Authorities (PCAs), each of which establishes and publishes its policies for registering users or organizations. Each PCA is certified by the IPRA. Below PCAs, CAs certify users and subordinate organizational entities (such as departments, offices, subsidiaries). Initially, the majority of users are expected to be registered with some organization. Some PCAs are expected to provide certification for users who wish to register independent of any organization. For users who wish anonymity while taking advantage of PEM privacy facilities, one or more PCAs are expected to be established with policies that allow for registration of users who do not wish to disclose their identities. PEM Messages PEM’s heart is its message format. Figure 24.4 shows an encrypted message using symmetric key management, Figure 24.5 shows an authenticated and encrypted message using public-key key management, and Figure 24.6 shows an authenticated (but unencrypted) message using public-key key management. The first field is “Proc-Type, ” and identifies the type of processing performed on the message. There are three possible types of messages. The “ENCRYPTED” specifier says that the message is encrypted and signed. The “MIC-ONLY” and “MIC-CLEAR” specifiers would indicate that the message is signed, but not encrypted. MIC-CLEAR messages are not encoded and can be read using non-PEM software. MIC-ONLY messages need PEM software to transform them to a human-readable form. A PEM message is always signed; it is optionally encrypted.
       -----BEGIN PRIVACY-ENHANCED MESSAGE-----
       Proc-Type: 4,ENCRYPTED
       Content-Domain: RFC822
       DEK-Info: DES-CBC,F8143EDE5960C597
       Originator-ID-Symmetric: schneier@counterpane.com,,
       Recipient-ID-Symmetric: schneier@chinet.com,ptf-kmc,3
       Key-Info:
    DES-ECB,RSA-MD2,9FD3AAD2F2691B9A,B70665BB9BF7CBCDA60195DB94F727D3
       Recipient-ID-Symmetric: pem-dev@tis.com,ptf-kmc,4
       Key-Info:
    DES-ECB,RSA-MD2,161A3F75DC82EF26,E2EF532C65CBCFF79F83A2658132DB47
    LLrHB0eJzyhP+/fSStdW8okeEnv47jxe7SJ/iN72ohNcUk2jHEUSoH1nvNSIWL9M
    8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIkjHUlBLpvXR0UrUzYbkNpk0agV2IzUpk
    J6UiRRGcDSvzrsoK+oNvqu6z7Xs5Xfz5rDqUcMlK1Z6720dcBWGGsDLpTpSCnpot
       dXd/H5LMDWnonNvPCwQUHt==
       -----END PRIVACY-ENHANCED MESSAGE-----
Figure 24.4 Example of an encapsulated message (symmetric case).
       -----BEGIN PRIVACY-ENHANCED MESSAGE-----
       Proc-Type: 4,ENCRYPTED
       Content-Domain: RFC822
       DEK-Info: DES-CBC,BFF968AA74691AC1
       Originator-Certificate:
    MIIBlTCCAScCAWUwDQYJKoZIhvcNAQECBQAwUTELMAkGA1UEBhMCVVMxIDAeBgNV
    BAoTF1JTQSBEYXRhIFNlY3VyaXR5LCBJbmMuMQ8wDQYDVQQLEwZCZXRhIDExDzAN
    BgNVBAsTBk5PVEFSWTAeFw05MTA5MDQxODM4MTdaFw05MzA5MDMxODM4MTZaMEUx
    CzAJBgNVBAYTAlVTMSAwHgYDVQQKExdSU0EgRGF0YSBTZWN1cml0eSwgSW5jLjEU
    MBIGA1UEAxMLVGVzdCBVc2VyIDEwWTAKBgRVCAEBAgICAANLADBIAkEAwHZHl7i+
    yJcqDtjJCowzTdBJrdAiLAnSC+CnnjOJELyuQiBgkGrgIh3j8/x0fM+YrsyF1u3F
    LZPVtzlndhYFJQIDAQABMA0GCSqGSIb3DQEBAgUAA1kACKr0PqphJYw1j+YPtcIq
    iWlFPuN5jJ79Khfg7ASFxskYkEMjRNZV/HZDZQEhtVaU7Jxfzs2wfX5byMp2X3U/
        5XUXGx7qusDgHQGs7Jk9W8CW1fuSWUgN4w==
       Key-Info: RSA,
    I3rRIGXUGWAF8js5wCzRTkdhO34PTHdRZY9Tuvm03M+NM7fx6qc5udixps2Lng0+
        wGrtiUm/ovtKdinz6ZQ/aQ==
       Issuer-Certificate:
    MIIB3DCCAUgCAQowDQYJKoZIhvcNAQECBQAwTzELMAkGA1UEBhMCVVMxIDAeBgNV
    BAoTF1JTQSBEYXRhIFNlY3VyaXR5LCBJbmMuMQ8wDQYDVQQLEwZCZXRhIDExDTAL
    BgNVBAsTBFRMQ0EwHhcNOTEwOTAxMDgwMDAwWhcNOTIwOTAxMDc1OTU5WjBRMQsw
    CQYDVQQGEwJVUzEgMB4GA1UEChMXUlNBIERhdGEgU2VjdXJpdHksIEluYy4xDzAN
    BgNVBAsTBkJldGEgMTEPMA0GA1UECxMGTk9UQVJZMHAwCgYEVQgBAQICArwDYgAw
    XwJYCsnp6lQCxYykNlODwutF/jMJ3kL+3PjYyHOwk+/9rLg6X65B/LD4bJHtO5XW
    cqAz/7R7XhjYCm0PcqbdzoACZtIlETrKrcJiDYoP+DkZ8k1gCk7hQHpbIwIDAQAB
    MA0GCSqGSIb3DQEBAgUAA38AAICPv4f9Gx/tY4+p+4DB7MV+tKZnvBoy8zgoMGOx
    dD2jMZ/3HsyWKWgSF0eH/AJB3qr9zosG47pyMnTf3aSy2nBO7CMxpUWRBcXUpE+x
    EREZd9++32ofGBIXaialnOgVUn0OzSYgugiQ077nJLDUj0hQehCizEs5wUJ35a5h
       MIC-Info: RSA-MD5,RSA,
    UdFJR8u/TIGhfH65ieewe21OW4tooa3vZCvVNGBZirf/7nrgzWDABz8w9NsXSexv
        AjRFbHoNPzBuxwmOAFeA0HJszL4yBvhG
       Recipient-ID-Asymmetric:
    MFExCzAJBgNVBAYTAlVTMSAwHgYDVQQKExdSU0EgRGF0YSBTZWN1cml0eSwgSW5j
        LjEPMA0GA1UECxMGQmV0YSAxMQ8wDQYDVQQLEwZOT1RBUlk=,
        66
       Key-Info: RSA,
    O6BS1ww9CTyHPtS3bMLD+L0hejdvX6Qv1HK2ds2sQPEaXhX8EhvVphHYTjwekdWv
        7x0Z3Jx2vTAhOYHMcqqCjA==
    qeWlj/YJ2Uf5ng9yznPbtD0mYloSwIuV9FRYx+gzY+8iXd/NQrXHfi6/MhPfPF3d
       jIqCJAxvld2xgqQimUzoS1a4r7kQQ5c/Iua4LqKeq3ciFzEv/MbZhA==
       -----END PRIVACY-ENHANCED MESSAGE-----
Figure 24.5 Example of an encapsulated encrypted message (asymmetric case). The next field, “Content-Domain, ” specifies the type of mail message. It has nothing to do with security. The “DEK-Info” field gives information on the Data Exchange Key (DEK), the encryption algorithm used to encrypt the text, and any parameters associated with the encryption algorithm. Only DES in CBC mode is currently specified, or “DES-CBC.” The second subfield specifies the IV. Other algorithms may be specified by PEM in the future; their use will be noted in DEK-Info and in other fields that identify algorithms. For messages with symmetric key management (see Figure 24.4), the next field is “Originator-ID-Symmetric” with three subfields. The first subfield identifies the sender by a unique electronic mail address. The second subfield is optional and identifies the authority that issued the interchange key. The third is an optional Version/Expiration subfield. Continuing with the symmetric key-management case, each recipient has two fields: “Recipient-ID-Symmetric” and “Key-Info.” The “Recipient-ID-Symmetric” field has three subfields; these identify the receiver in the same way that “Originator-ID-Symmetric” identified the sender. The “Key-Info” field specifies the key-management parameters. This field has four subfields. The first subfield gives the algorithm used to encrypt the DEK. Since the key management in this message is symmetric, the sender and receiver have to share a common key. This is called the Interchange Key (IK), which is used to encrypt the DEK. The DEK can be either encrypted using DES in ECB (denoted by “DES-ECB”) or triple-DES (which would be denoted “DES-EDE”). The second subfield specifies the MIC algorithm. It can be either MD2 (denoted by “RSA-MD2”) or MD5 (which would be denoted “RSA-MD5”). The third subfield, the DEK, and the fourth field, the MIC, are both encrypted with the IK.
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