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pgpfan:pgpauth [2025/10/10 21:53] – [The PGP OCFB Encryption Mode] Better explanation order. b.walzerpgpfan:pgpauth [2025/11/01 11:57] (current) – [The Why of PGP Authentication] Typo b.walzer
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-======The Why of PGP Authentication======+ ======The Why of PGP Authentication======
  
 There seems to be an ongoing misunderstanding of how PGP actually does authentication. That ends up being important in various discussions. Let's dig into perhaps the more interesting question of //why// PGP does the things it does and pick up the //how// along the way. There seems to be an ongoing misunderstanding of how PGP actually does authentication. That ends up being important in various discussions. Let's dig into perhaps the more interesting question of //why// PGP does the things it does and pick up the //how// along the way.
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 Authentication in a PGP context is a mechanism that makes it possible for the recipient of a file/message to verify that the file/message was certified by a particular entity/person. It also insures that the file/message was not tampered with after that certification. The most common convention is that the certification is a declaration that the certifier created a particular file/message. More generally, such a certification means that the certifier is taking responsibility for the file/message in some way. Authentication in a PGP context is a mechanism that makes it possible for the recipient of a file/message to verify that the file/message was certified by a particular entity/person. It also insures that the file/message was not tampered with after that certification. The most common convention is that the certification is a declaration that the certifier created a particular file/message. More generally, such a certification means that the certifier is taking responsibility for the file/message in some way.
  
-Cryptographic signatures will be important here so we should briefly define the term. The generation of a [[wp>Digital_signature|cryptographic signature]] requires knowledge of a secret value normally only known by single entity/person. There is a public value that corresponds to the secret value. Anyone can take that public value, the signed data and the cryptographic signature and verify that the signer had access to the secret value when they created the cryptographic signature. Cryptographic signatures often form the root of authentication schemes.+Cryptographic signatures will be important here so we should briefly define the term. The generation of a [[wp>Digital_signature|cryptographic signature]] requires knowledge of a secret value normally only known by a single entity/person. There is a public value that corresponds to the secret value. Anyone can take that public value, the signed data and the cryptographic signature and verify that the signer had access to the secret value when they created the cryptographic signature. Cryptographic signatures often form the root of authentication schemes.
  
 =====PGP Authentication Doesn't Work This Way===== =====PGP Authentication Doesn't Work This Way=====
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 =====The PGP OCFB Encryption Mode===== =====The PGP OCFB Encryption Mode=====
 +
 +The very existence of this mode seems to confuse people...
  
 This is an encryption mode with no integrity check((The PGP OCFB mode is contained in the OpenPGP Symmetrical Encryption Data packet (SED).)). It is pure encryption. We should have a pretty good idea at this point why it exists. Since PGP authentication is done directly on the data there is simply no need for an integrity check. An encryption mode that provided an integrity check feature would be redundant and would cause inefficiency. This is an encryption mode with no integrity check((The PGP OCFB mode is contained in the OpenPGP Symmetrical Encryption Data packet (SED).)). It is pure encryption. We should have a pretty good idea at this point why it exists. Since PGP authentication is done directly on the data there is simply no need for an integrity check. An encryption mode that provided an integrity check feature would be redundant and would cause inefficiency.
  
-The very existence of this mode seems to confuse people... 
  
 =====The PGP OCFB-MDC Encryption Mode===== =====The PGP OCFB-MDC Encryption Mode=====
  
-So there is a PGP encryption mode //with// an integrity check((The PGP OCFB-MDC mode is contained in the OpenPGP Symmetrical Encryption Integrity Protected Data packet (SEIPD).)). For those of us who know how PGP authentication works the existence of //this// mode is confusing.+So... This is awkward... 
 + 
 +OCFB-MDC is a PGP encryption mode //with// an integrity check((The PGP OCFB-MDC mode is contained in the OpenPGP Symmetrical Encryption Integrity Protected Data packet (SEIPD).)). For those of us who know how PGP authentication works the existence of //this// mode is confusing.
  
 People were happily and securely using the OCFB encryption mode with PGP authentication for many years. The OCFB-MDC mode came along later. People were happily and securely using the OCFB encryption mode with PGP authentication for many years. The OCFB-MDC mode came along later.
  
-A valid OCFB-MDC integrity check insures:+A valid OCFB-MDC integrity check ensures:
  
   - The file/message has not been modified.   - The file/message has not been modified.
  
-Since there is nothing like the "TLS handshake" to link authentication to the integrity check we don't get any assurance that the file/message has not been created by a third party. Oh, and just for the record, none of the proposed replacements for OCFB-MDC (at least four at the time this was written) do anything different. They only provide an integrity check, not any sort of authentication.+Since there is nothing like the "TLS handshake" to link authentication to the integrity check we don't get any assurance that the file/message has not been created by a third party as we would if we were using authentication. An attacker can create whatever malicious message they want and then send it to you anonymously. An attacker can create a whole new file and overwrite the one that exists. The integrity check will pass in both cases. Oh, and just for the record, none of the proposed replacements for OCFB-MDC (at least four at the time this was written) do anything different. They only provide an integrity check, not any sort of authentication.
  
 So why does the OCFB-MDC encryption mode exist at all? It's specifically for the case of anonymous (unsigned, unauthenticated) files/messages. We won't know where the file/message came from, but at least we know it wasn't tampered with. It's better than nothing. So why does the OCFB-MDC encryption mode exist at all? It's specifically for the case of anonymous (unsigned, unauthenticated) files/messages. We won't know where the file/message came from, but at least we know it wasn't tampered with. It's better than nothing.
  
-In practice almost all implementations will emit OCFB-MDC encrypted messages even if the file/message is signed. That makes it so that schemes that depend on the integrity check will always have it available without consideration of the authentication status of the message. The resulting inefficiency is apparently considered an acceptable cost. Those implementations that do not require the integrity check can simply just ignore it.+In practice almost all PGP implementations will emit OCFB-MDC encrypted messages even if the file/message is signed. That makes it so that schemes that depend on the integrity check will always have it available without consideration of the authentication status of the message. The resulting inefficiency is apparently considered an acceptable cost. Those implementations that do not require the integrity check can simply just ignore it.
  
 When working with PGP, it is important to clearly understand the difference between authentication and the integrity check. The integrity check is limited and often full authentication is required or more desirable. When working with PGP, it is important to clearly understand the difference between authentication and the integrity check. The integrity check is limited and often full authentication is required or more desirable.
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pgpfan/pgpauth.1760133199.txt.gz · Last modified: by b.walzer