base64Binary vs hexBinary in XML Schema

In an XML Schema (XSD), both xs:base64Binary and xs:hexBinary are built-in datatypes used to embed arbitrary binary data into XML documents as text. The primary differences between them lie in their underlying encoding algorithms, the resulting data size overhead, and their typical use cases: hexBinary encodes each byte into two hexadecimal characters resulting in a 100% size increase, while base64Binary encodes every three bytes into four ASCII characters resulting in approximately a 33% size increase.

Encoding Format and Character Sets

• xs:hexBinary: Uses hexadecimal representation. Every byte (8 bits) of binary data is represented by two hexadecimal digits. The allowed character set consists of numbers 0-9 and letters A-F (case-insensitive, though uppercase is the canonical format).
• xs:base64Binary: Uses the Base64 encoding scheme defined in RFC 2045. It breaks binary data into 6-bit chunks, mapping each chunk to a 64-character alphabet (A-Z, a-z, 0-9, +, and /), using = for padding.

Size and Overhead

• xs:hexBinary Efficiency: Expands data by a factor of 2 (100% overhead). For example, a 3-byte binary sequence requires 6 characters in XML.
• xs:base64Binary Efficiency: Expands data by a factor of approximately 1.33 (33% overhead). A 3-byte binary sequence requires only 4 characters in XML.

Because base64Binary produces significantly smaller payloads, it is substantially more bandwidth- and memory-efficient for transmitting binary data.

Readability and Processing

• xs:hexBinary: Offers better human readability and straightforward parsing when dealing with raw byte sequences, memory dumps, or fixed-length hashes.
• xs:base64Binary: Is less readable to human inspection due to the 6-bit grouping, but it is the universal standard for embedding media and files across web technologies.

When to Use Which

• Use xs:hexBinary for small, fixed-length values where hexadecimal representation is conventional, such as:
  • Cryptographic hashes (e.g., SHA-256, MD5)
  • MAC addresses and UUIDs
  • Color codes and bitmasks
• Use xs:base64Binary for larger, arbitrary binary streams to minimize document size, such as:
  • Embedded images, audio, or video files
  • PDF and office documents
  • Digital signatures and raw encrypted payloads