To secure the authentication of computers using IPsec, two types of encryption protocols that can be used are SHA (Secure Hash Algorithm) and MD5 (Message Digest Algorithm 5).

1. SHA (Secure Hash Algorithm):
- SHA is a widely used cryptographic hash function that generates a fixed-size hash value from input data.
- It provides a high level of security and is designed to be collision-resistant, meaning it is computationally infeasible to find two different inputs that produce the same hash output.
- IPsec can use SHA-1, SHA-256, or SHA-384 for hashing purposes.
- SHA-1 is the most commonly used version, but it is gradually being phased out due to security vulnerabilities.
- SHA-256 and SHA-384 offer stronger security and are recommended for use in IPsec.

2. MD5 (Message Digest Algorithm 5):
- MD5 is a widely used cryptographic hash function that takes an input (message) and produces a fixed-size 128-bit hash value.
- It is faster than SHA, but it is considered to be less secure due to its vulnerability to collision attacks.
- Collision attacks involve finding two different inputs that produce the same hash value, which could potentially allow an attacker to impersonate another entity or modify data without detection.
- As a result, MD5 is not recommended for use in security-critical applications such as IPsec.
- However, it may still be used for non-security-critical purposes where speed is a priority, such as checksumming or checksum-based integrity checks.

In the context of securing the authentication of computers using IPsec, SHA is the preferred choice for hashing purposes because of its stronger security properties. MD5, on the other hand, should be avoided due to its vulnerabilities. Therefore, the correct answer is option 'C,D' (SHA and MD5).