2. Bits, bytes, and encoding

Treat a byte as 8 bits with values from 0 to 255, also called an octet. Count byte lengths and recognize why cryptographic keys, messages, and ciphertexts are measured in bytes.

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Convert bytes into two hexadecimal digits using 4-bit nibbles, and read common forms like 0x2f or spaced byte dumps. Use hex to inspect binary data without writing long binary strings.

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Work with byte arrays as ordered sequences of byte values that can be indexed, sliced, copied, and concatenated. Recognize the difference between one byte value and a multi-byte byte string.

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Separate raw bytes from the text used to display or transmit them. Compare a real byte like 0x41 with the characters “41” so hex dumps, logs, and protocol examples do not get confused with the underlying data.

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Map visible characters to Unicode code points and then to UTF-8 bytes. Encode simple ASCII text and a non-ASCII character to see why character count and byte length can differ.

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Use Base64 as a compact text form for arbitrary bytes in files, APIs, and test vectors. Decode how padding and alphabet choices affect the displayed string while leaving the underlying bytes unchanged.

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Turn small integers into fixed numbers of bytes and read them back using big-endian or little-endian order. Compare the same value in both byte orders so protocol specifications become unambiguous.

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Parse a byte string by using agreed field sizes, length prefixes, or concatenation rules. Split a toy public key or ciphertext layout into named byte ranges and check that the total length matches the format.

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Apply the XOR truth table to bits and then XOR equal-length byte arrays byte by byte. Trace a tiny message, key, and ciphertext example to see why c = m XOR k and m = c XOR k use the same operation.

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Use what you learned in the previous lesson to solve real-world problems.

Review this chapter with practice based on your mistakes.