base64/engine/
mod.rs

1//! Provides the [Engine] abstraction and out of the box implementations.
2#[cfg(any(feature = "alloc", test))]
3use crate::chunked_encoder;
4use crate::{
5    encode::{encode_with_padding, EncodeSliceError},
6    encoded_len, DecodeError, DecodeSliceError,
7};
8#[cfg(any(feature = "alloc", test))]
9use alloc::vec::Vec;
10
11#[cfg(any(feature = "alloc", test))]
12use alloc::{string::String, vec};
13
14pub mod general_purpose;
15
16#[cfg(test)]
17mod naive;
18
19#[cfg(test)]
20mod tests;
21
22pub use general_purpose::{GeneralPurpose, GeneralPurposeConfig};
23
24/// An `Engine` provides low-level encoding and decoding operations that all other higher-level parts of the API use. Users of the library will generally not need to implement this.
25///
26/// Different implementations offer different characteristics. The library currently ships with
27/// [GeneralPurpose] that offers good speed and works on any CPU, with more choices
28/// coming later, like a constant-time one when side channel resistance is called for, and vendor-specific vectorized ones for more speed.
29///
30/// See [general_purpose::STANDARD_NO_PAD] if you just want standard base64. Otherwise, when possible, it's
31/// recommended to store the engine in a `const` so that references to it won't pose any lifetime
32/// issues, and to avoid repeating the cost of engine setup.
33///
34/// Since almost nobody will need to implement `Engine`, docs for internal methods are hidden.
35// When adding an implementation of Engine, include them in the engine test suite:
36// - add an implementation of [engine::tests::EngineWrapper]
37// - add the implementation to the `all_engines` macro
38// All tests run on all engines listed in the macro.
39pub trait Engine: Send + Sync {
40    /// The config type used by this engine
41    type Config: Config;
42    /// The decode estimate used by this engine
43    type DecodeEstimate: DecodeEstimate;
44
45    /// This is not meant to be called directly; it is only for `Engine` implementors.
46    /// See the other `encode*` functions on this trait.
47    ///
48    /// Encode the `input` bytes into the `output` buffer based on the mapping in `encode_table`.
49    ///
50    /// `output` will be long enough to hold the encoded data.
51    ///
52    /// Returns the number of bytes written.
53    ///
54    /// No padding should be written; that is handled separately.
55    ///
56    /// Must not write any bytes into the output slice other than the encoded data.
57    #[doc(hidden)]
58    fn internal_encode(&self, input: &[u8], output: &mut [u8]) -> usize;
59
60    /// This is not meant to be called directly; it is only for `Engine` implementors.
61    ///
62    /// As an optimization to prevent the decoded length from being calculated twice, it is
63    /// sometimes helpful to have a conservative estimate of the decoded size before doing the
64    /// decoding, so this calculation is done separately and passed to [Engine::decode()] as needed.
65    #[doc(hidden)]
66    fn internal_decoded_len_estimate(&self, input_len: usize) -> Self::DecodeEstimate;
67
68    /// This is not meant to be called directly; it is only for `Engine` implementors.
69    /// See the other `decode*` functions on this trait.
70    ///
71    /// Decode `input` base64 bytes into the `output` buffer.
72    ///
73    /// `decode_estimate` is the result of [Engine::internal_decoded_len_estimate()], which is passed in to avoid
74    /// calculating it again (expensive on short inputs).`
75    ///
76    /// Each complete 4-byte chunk of encoded data decodes to 3 bytes of decoded data, but this
77    /// function must also handle the final possibly partial chunk.
78    /// If the input length is not a multiple of 4, or uses padding bytes to reach a multiple of 4,
79    /// the trailing 2 or 3 bytes must decode to 1 or 2 bytes, respectively, as per the
80    /// [RFC](https://tools.ietf.org/html/rfc4648#section-3.5).
81    ///
82    /// Decoding must not write any bytes into the output slice other than the decoded data.
83    ///
84    /// Non-canonical trailing bits in the final tokens or non-canonical padding must be reported as
85    /// errors unless the engine is configured otherwise.
86    ///
87    /// # Panics
88    ///
89    /// Panics if `output` is too small.
90    #[doc(hidden)]
91    fn internal_decode(
92        &self,
93        input: &[u8],
94        output: &mut [u8],
95        decode_estimate: Self::DecodeEstimate,
96    ) -> Result<DecodeMetadata, DecodeError>;
97
98    /// Returns the config for this engine.
99    fn config(&self) -> &Self::Config;
100
101    /// Encode arbitrary octets as base64 using the provided `Engine`.
102    /// Returns a `String`.
103    ///
104    /// # Example
105    ///
106    /// ```rust
107    /// use base64::{Engine as _, engine::{self, general_purpose}, alphabet};
108    ///
109    /// let b64 = general_purpose::STANDARD.encode(b"hello world~");
110    /// println!("{}", b64);
111    ///
112    /// const CUSTOM_ENGINE: engine::GeneralPurpose =
113    ///     engine::GeneralPurpose::new(&alphabet::URL_SAFE, general_purpose::NO_PAD);
114    ///
115    /// let b64_url = CUSTOM_ENGINE.encode(b"hello internet~");
116    #[cfg(any(feature = "alloc", test))]
117    #[inline]
118    fn encode<T: AsRef<[u8]>>(&self, input: T) -> String {
119        fn inner<E>(engine: &E, input_bytes: &[u8]) -> String
120        where
121            E: Engine + ?Sized,
122        {
123            let encoded_size = encoded_len(input_bytes.len(), engine.config().encode_padding())
124                .expect("integer overflow when calculating buffer size");
125
126            let mut buf = vec![0; encoded_size];
127
128            encode_with_padding(input_bytes, &mut buf[..], engine, encoded_size);
129
130            String::from_utf8(buf).expect("Invalid UTF8")
131        }
132
133        inner(self, input.as_ref())
134    }
135
136    /// Encode arbitrary octets as base64 into a supplied `String`.
137    /// Writes into the supplied `String`, which may allocate if its internal buffer isn't big enough.
138    ///
139    /// # Example
140    ///
141    /// ```rust
142    /// use base64::{Engine as _, engine::{self, general_purpose}, alphabet};
143    /// const CUSTOM_ENGINE: engine::GeneralPurpose =
144    ///     engine::GeneralPurpose::new(&alphabet::URL_SAFE, general_purpose::NO_PAD);
145    ///
146    /// fn main() {
147    ///     let mut buf = String::new();
148    ///     general_purpose::STANDARD.encode_string(b"hello world~", &mut buf);
149    ///     println!("{}", buf);
150    ///
151    ///     buf.clear();
152    ///     CUSTOM_ENGINE.encode_string(b"hello internet~", &mut buf);
153    ///     println!("{}", buf);
154    /// }
155    /// ```
156    #[cfg(any(feature = "alloc", test))]
157    #[inline]
158    fn encode_string<T: AsRef<[u8]>>(&self, input: T, output_buf: &mut String) {
159        fn inner<E>(engine: &E, input_bytes: &[u8], output_buf: &mut String)
160        where
161            E: Engine + ?Sized,
162        {
163            let mut sink = chunked_encoder::StringSink::new(output_buf);
164
165            chunked_encoder::ChunkedEncoder::new(engine)
166                .encode(input_bytes, &mut sink)
167                .expect("Writing to a String shouldn't fail");
168        }
169
170        inner(self, input.as_ref(), output_buf)
171    }
172
173    /// Encode arbitrary octets as base64 into a supplied slice.
174    /// Writes into the supplied output buffer.
175    ///
176    /// This is useful if you wish to avoid allocation entirely (e.g. encoding into a stack-resident
177    /// or statically-allocated buffer).
178    ///
179    /// # Example
180    ///
181    #[cfg_attr(feature = "alloc", doc = "```")]
182    #[cfg_attr(not(feature = "alloc"), doc = "```ignore")]
183    /// use base64::{Engine as _, engine::general_purpose};
184    /// let s = b"hello internet!";
185    /// let mut buf = Vec::new();
186    /// // make sure we'll have a slice big enough for base64 + padding
187    /// buf.resize(s.len() * 4 / 3 + 4, 0);
188    ///
189    /// let bytes_written = general_purpose::STANDARD.encode_slice(s, &mut buf).unwrap();
190    ///
191    /// // shorten our vec down to just what was written
192    /// buf.truncate(bytes_written);
193    ///
194    /// assert_eq!(s, general_purpose::STANDARD.decode(&buf).unwrap().as_slice());
195    /// ```
196    #[inline]
197    fn encode_slice<T: AsRef<[u8]>>(
198        &self,
199        input: T,
200        output_buf: &mut [u8],
201    ) -> Result<usize, EncodeSliceError> {
202        fn inner<E>(
203            engine: &E,
204            input_bytes: &[u8],
205            output_buf: &mut [u8],
206        ) -> Result<usize, EncodeSliceError>
207        where
208            E: Engine + ?Sized,
209        {
210            let encoded_size = encoded_len(input_bytes.len(), engine.config().encode_padding())
211                .expect("usize overflow when calculating buffer size");
212
213            if output_buf.len() < encoded_size {
214                return Err(EncodeSliceError::OutputSliceTooSmall);
215            }
216
217            let b64_output = &mut output_buf[0..encoded_size];
218
219            encode_with_padding(input_bytes, b64_output, engine, encoded_size);
220
221            Ok(encoded_size)
222        }
223
224        inner(self, input.as_ref(), output_buf)
225    }
226
227    /// Decode the input into a new `Vec`.
228    ///
229    /// # Example
230    ///
231    /// ```rust
232    /// use base64::{Engine as _, alphabet, engine::{self, general_purpose}};
233    ///
234    /// let bytes = general_purpose::STANDARD
235    ///     .decode("aGVsbG8gd29ybGR+Cg==").unwrap();
236    /// println!("{:?}", bytes);
237    ///
238    /// // custom engine setup
239    /// let bytes_url = engine::GeneralPurpose::new(
240    ///              &alphabet::URL_SAFE,
241    ///              general_purpose::NO_PAD)
242    ///     .decode("aGVsbG8gaW50ZXJuZXR-Cg").unwrap();
243    /// println!("{:?}", bytes_url);
244    /// ```
245    #[cfg(any(feature = "alloc", test))]
246    #[inline]
247    fn decode<T: AsRef<[u8]>>(&self, input: T) -> Result<Vec<u8>, DecodeError> {
248        fn inner<E>(engine: &E, input_bytes: &[u8]) -> Result<Vec<u8>, DecodeError>
249        where
250            E: Engine + ?Sized,
251        {
252            let estimate = engine.internal_decoded_len_estimate(input_bytes.len());
253            let mut buffer = vec![0; estimate.decoded_len_estimate()];
254
255            let bytes_written = engine
256                .internal_decode(input_bytes, &mut buffer, estimate)?
257                .decoded_len;
258
259            buffer.truncate(bytes_written);
260
261            Ok(buffer)
262        }
263
264        inner(self, input.as_ref())
265    }
266
267    /// Decode the `input` into the supplied `buffer`.
268    ///
269    /// Writes into the supplied `Vec`, which may allocate if its internal buffer isn't big enough.
270    /// Returns a `Result` containing an empty tuple, aka `()`.
271    ///
272    /// # Example
273    ///
274    /// ```rust
275    /// use base64::{Engine as _, alphabet, engine::{self, general_purpose}};
276    /// const CUSTOM_ENGINE: engine::GeneralPurpose =
277    ///     engine::GeneralPurpose::new(&alphabet::URL_SAFE, general_purpose::PAD);
278    ///
279    /// fn main() {
280    ///     use base64::Engine;
281    ///     let mut buffer = Vec::<u8>::new();
282    ///     // with the default engine
283    ///     general_purpose::STANDARD
284    ///         .decode_vec("aGVsbG8gd29ybGR+Cg==", &mut buffer,).unwrap();
285    ///     println!("{:?}", buffer);
286    ///
287    ///     buffer.clear();
288    ///
289    ///     // with a custom engine
290    ///     CUSTOM_ENGINE.decode_vec(
291    ///         "aGVsbG8gaW50ZXJuZXR-Cg==",
292    ///         &mut buffer,
293    ///     ).unwrap();
294    ///     println!("{:?}", buffer);
295    /// }
296    /// ```
297    #[cfg(any(feature = "alloc", test))]
298    #[inline]
299    fn decode_vec<T: AsRef<[u8]>>(
300        &self,
301        input: T,
302        buffer: &mut Vec<u8>,
303    ) -> Result<(), DecodeError> {
304        fn inner<E>(engine: &E, input_bytes: &[u8], buffer: &mut Vec<u8>) -> Result<(), DecodeError>
305        where
306            E: Engine + ?Sized,
307        {
308            let starting_output_len = buffer.len();
309            let estimate = engine.internal_decoded_len_estimate(input_bytes.len());
310
311            let total_len_estimate = estimate
312                .decoded_len_estimate()
313                .checked_add(starting_output_len)
314                .expect("Overflow when calculating output buffer length");
315
316            buffer.resize(total_len_estimate, 0);
317
318            let buffer_slice = &mut buffer.as_mut_slice()[starting_output_len..];
319
320            let bytes_written = engine
321                .internal_decode(input_bytes, buffer_slice, estimate)?
322                .decoded_len;
323
324            buffer.truncate(starting_output_len + bytes_written);
325
326            Ok(())
327        }
328
329        inner(self, input.as_ref(), buffer)
330    }
331
332    /// Decode the input into the provided output slice.
333    ///
334    /// Returns the number of bytes written to the slice, or an error if `output` is smaller than
335    /// the estimated decoded length.
336    ///
337    /// This will not write any bytes past exactly what is decoded (no stray garbage bytes at the end).
338    ///
339    /// See [crate::decoded_len_estimate] for calculating buffer sizes.
340    ///
341    /// See [Engine::decode_slice_unchecked] for a version that panics instead of returning an error
342    /// if the output buffer is too small.
343    #[inline]
344    fn decode_slice<T: AsRef<[u8]>>(
345        &self,
346        input: T,
347        output: &mut [u8],
348    ) -> Result<usize, DecodeSliceError> {
349        fn inner<E>(
350            engine: &E,
351            input_bytes: &[u8],
352            output: &mut [u8],
353        ) -> Result<usize, DecodeSliceError>
354        where
355            E: Engine + ?Sized,
356        {
357            let estimate = engine.internal_decoded_len_estimate(input_bytes.len());
358
359            if output.len() < estimate.decoded_len_estimate() {
360                return Err(DecodeSliceError::OutputSliceTooSmall);
361            }
362
363            engine
364                .internal_decode(input_bytes, output, estimate)
365                .map_err(|e| e.into())
366                .map(|dm| dm.decoded_len)
367        }
368
369        inner(self, input.as_ref(), output)
370    }
371
372    /// Decode the input into the provided output slice.
373    ///
374    /// Returns the number of bytes written to the slice.
375    ///
376    /// This will not write any bytes past exactly what is decoded (no stray garbage bytes at the end).
377    ///
378    /// See [crate::decoded_len_estimate] for calculating buffer sizes.
379    ///
380    /// See [Engine::decode_slice] for a version that returns an error instead of panicking if the output
381    /// buffer is too small.
382    ///
383    /// # Panics
384    ///
385    /// Panics if the provided output buffer is too small for the decoded data.
386    #[inline]
387    fn decode_slice_unchecked<T: AsRef<[u8]>>(
388        &self,
389        input: T,
390        output: &mut [u8],
391    ) -> Result<usize, DecodeError> {
392        fn inner<E>(engine: &E, input_bytes: &[u8], output: &mut [u8]) -> Result<usize, DecodeError>
393        where
394            E: Engine + ?Sized,
395        {
396            engine
397                .internal_decode(
398                    input_bytes,
399                    output,
400                    engine.internal_decoded_len_estimate(input_bytes.len()),
401                )
402                .map(|dm| dm.decoded_len)
403        }
404
405        inner(self, input.as_ref(), output)
406    }
407}
408
409/// The minimal level of configuration that engines must support.
410pub trait Config {
411    /// Returns `true` if padding should be added after the encoded output.
412    ///
413    /// Padding is added outside the engine's encode() since the engine may be used
414    /// to encode only a chunk of the overall output, so it can't always know when
415    /// the output is "done" and would therefore need padding (if configured).
416    // It could be provided as a separate parameter when encoding, but that feels like
417    // leaking an implementation detail to the user, and it's hopefully more convenient
418    // to have to only pass one thing (the engine) to any part of the API.
419    fn encode_padding(&self) -> bool;
420}
421
422/// The decode estimate used by an engine implementation. Users do not need to interact with this;
423/// it is only for engine implementors.
424///
425/// Implementors may store relevant data here when constructing this to avoid having to calculate
426/// them again during actual decoding.
427pub trait DecodeEstimate {
428    /// Returns a conservative (err on the side of too big) estimate of the decoded length to use
429    /// for pre-allocating buffers, etc.
430    ///
431    /// The estimate must be no larger than the next largest complete triple of decoded bytes.
432    /// That is, the final quad of tokens to decode may be assumed to be complete with no padding.
433    fn decoded_len_estimate(&self) -> usize;
434}
435
436/// Controls how pad bytes are handled when decoding.
437///
438/// Each [Engine] must support at least the behavior indicated by
439/// [DecodePaddingMode::RequireCanonical], and may support other modes.
440#[derive(Clone, Copy, Debug, PartialEq, Eq)]
441pub enum DecodePaddingMode {
442    /// Canonical padding is allowed, but any fewer padding bytes than that is also allowed.
443    Indifferent,
444    /// Padding must be canonical (0, 1, or 2 `=` as needed to produce a 4 byte suffix).
445    RequireCanonical,
446    /// Padding must be absent -- for when you want predictable padding, without any wasted bytes.
447    RequireNone,
448}
449
450/// Metadata about the result of a decode operation
451#[derive(PartialEq, Eq, Debug)]
452pub struct DecodeMetadata {
453    /// Number of decoded bytes output
454    pub(crate) decoded_len: usize,
455    /// Offset of the first padding byte in the input, if any
456    pub(crate) padding_offset: Option<usize>,
457}
458
459impl DecodeMetadata {
460    pub(crate) fn new(decoded_bytes: usize, padding_index: Option<usize>) -> Self {
461        Self {
462            decoded_len: decoded_bytes,
463            padding_offset: padding_index,
464        }
465    }
466}
base64/engine/mod.rs

base64/engine/
mod.rs
