Some IPsec references
Some IPsec ressources I was using while developing ipdecap
RFCs
 Requirement    Encryption Algorithm (notes)
        -----------    --------------------------
        MUST           NULL [RFC2410] (1)
        MUST           AES-CBC with 128-bit keys [RFC3602]
        MUST-          TripleDES-CBC [RFC2451]
        SHOULD         AES-CTR [RFC3686]
        SHOULD NOT     DES-CBC [RFC2405] (2)
        Requirement    Authentication Algorithm (notes)
        -----------    -----------------------------
        MUST           HMAC-SHA1-96 [RFC2404] (3)
        SHOULD+        AES-XCBC-MAC-96 [RFC3566]
        MAY            NULL (1)
        MAY            HMAC-MD5-96 [RFC2403] (4)
Authentication
- rfc 2104: HMAC: Keyed-Hashing for Message Authentication
 - rfc 2403: The Use of HMAC-MD5-96 within ESP and AH
 - rfc 2404: The Use of HMAC-SHA-1-96 within ESP and AH
 - rfc 2410: The NULL Encryption Algorithm and Its Use With IPsec
 - rfc 4868: Using HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 with IPsec
 - draft: The HMAC-SHA-256-96 Algorithm and Its Use With IPsec
 - draft: The HMAC-SHA-256-128 Algorithm and Its Use With IPsec
 
Encryption
- rfc 2405: The ESP DES-CBC Cipher Algorithm With Explicit IV
 - rfc 3686: Using Advanced Encryption Standard (AES) Counter Mode With IPsec Encapsulating Security Payload (ESP)
 - rfc 3602: The AES-CBC Cipher Algorithm and Its Use with IPsec
 - draft: The ESP CAST5-128-CBC Transform
 - draft: The ESP Blowfish-CBC Algorithm Using an Explicit IV
 - draft: The ESP CAST128-CBC Algorithm
 - draft: The ESP RC5-CBC Algorithm