README.md

Encryption CTFs

"Padding" CTF

Challenge Description

Disable the padding in the encryption process.

Key Instructions

1. Use the EVP_CIPHER_CTX_set_padding(ctx, 0); function to disable padding.
2. Surround the instruction with CRYPTO25{} to obtain the flag.

Example Code

Refer to the enc1.c file for the implementation details.

"First Decryption" CTF

Challenge Description

Decrypt a Base64 encoded message using the specified key and IV.

Key Instructions

1. Decode the Base64 encoded message.
2. Use the EVP_CipherInit function to initialize the decryption operation.
3. Decrypt the ciphertext and print the decrypted text as the flag.

Example Code

Refer to the decryption.c file for the implementation details.

"In the Name of the Cipher" CTF

Challenge Description

Encrypt the content of a file using a user-selected algorithm.

Key Instructions

1. Read the input file, key, IV, output file, and algorithm from the command line.
2. Use the EVP_get_cipherbyname function to get the cipher by name.
3. Encrypt the file content using the specified algorithm.
4. Print the flag as CRYPTO25{EVP_get_cipherbyname}.

Example Code

Refer to the encryption.c file for the implementation details.

"Guess Algo" CTF

Challenge Description

Decrypt a Base64 encoded string using the specified key and IV.

Key Instructions

1. Decode the Base64 encoded string.
2. Try different algorithms to decrypt the ciphertext.
3. Check if the decrypted content is valid ASCII text.
4. Print the flag as CRYPTO25{decryptedcontentalgorithm_name}.

Example Code

Refer to the decryption.c file for the implementation details.

Additional Resources

• OpenSSL Encryption Documentation
• Base64 Encoding