Size of Key Material of TLS_DHE_RSA_WITH_AES_256_CBC_SHA

Question

What is the size of key material required for generating MAC keys, encryption keys and IV when use TLS_DHE_RSA_WITH_AES_256_CBC_SHA ? How to calculate it?

Answer 1

TLS_DHE_RSA_WITH_AES_256_CBC_SHA is

• 256-bit AES encryption
• SHA-1 message authentication
• Ephemeral Diffie-Hellman key exchange
• Signed with an RSA certificate

We can find the answer in rfc5246

                        Key      IV   Block
Cipher        Type    Material  Size  Size
------------  ------  --------  ----  -----
NULL          Stream      0       0    N/A
RC4_128       Stream     16       0    N/A
3DES_EDE_CBC  Block      24       8      8
AES_128_CBC   Block      16      16     16
AES_256_CBC   Block      32      16     16
MAC       Algorithm    mac_length  mac_key_length

NULL      N/A              0             0
MD5       HMAC-MD5        16            16
SHA       HMAC-SHA1       20            20
SHA256    HMAC-SHA256     32            32

• Note 1: the values in bytes.
• Note 2: This mode is archaic and exist in TLS since TLS1.0. You should stick to TLS 1.3 which has huge cleanup and has only 5 cipher suites and all use Authenticated Encryption with Associated Data (AEAD).