Re: [quicwg/base-drafts] QPACK security considerations (#3575)

[Lucas Pardue <[e-mail]>]

@LPardue approved this pull request.

lgtm modulo some nits

> +confirm guesses a character at a time, reducing an exponential-time attack into
+a linear-time attack.
+
+## Applicability to QPACK and HTTP
+
+QPACK mitigates but does not completely prevent attacks modeled on CRIME [CRIME]
+by forcing a guess to match an entire header field value, rather than individual
+characters. An attacker can only learn whether a guess is correct or not, so is
+reduced to a brute force guess for the header field values.
+
+The viability of recovering specific header field values therefore depends on
+the entropy of values. As a result, values with high entropy are unlikely to be
+recovered successfully. However, values with low entropy remain vulnerable.
+
+Attacks of this nature are possible any time that two mutually distrustful
+entities control requests or responses that are placed onto a single HTTP/2

```suggestion
entities control requests or responses that are placed onto a single HTTP/3
```

> +The amount of memory used by the compressor is limited by the protocol using
+QPACK through the definition of the maximum size of the dynamic table, and the
+maximum number of blocking streams. In HTTP/3, these values are controlled by
+the decoder through the setting parameter QPACK_MAX_TABLE_CAPACITY and
+QPACK_BLOCKED_STREAMS, respectively (see Section
+{{maximum-dynamic-table-capacity}} and {{blocked-streams}}). The limit on the
+size of the dynamic table takes into account both the size of the data stored in
+the dynamic table, plus a small allowance for overhead.  The limit on the number
+of blocked streams is only a proxy for the maximum amount of memory required by
+the decoder.  The actual maximum amount of memory will depend on how much memory
+the decoder uses to track each blocked stream.
+
+A decoder can limit the amount of state memory used for the dynamic table by
+setting an appropriate value for the maximum size of the dynamic table. In
+HTTP/3, this is realized by setting an appropriate value for the
+QPACK_MAX_TABLE_CAPACITY parameter. An encoder can limit the amount of state

nit: the doc is inconsistent about QPACK_MAX_TABLE_CAPACITY  vs SETTINGS_QPACK_MAX_TABLE_CAPACITY. I don't know what the answer is

> +QPACK through the definition of the maximum size of the dynamic table, and the
+maximum number of blocking streams. In HTTP/3, these values are controlled by
+the decoder through the setting parameter QPACK_MAX_TABLE_CAPACITY and
+QPACK_BLOCKED_STREAMS, respectively (see Section
+{{maximum-dynamic-table-capacity}} and {{blocked-streams}}). The limit on the
+size of the dynamic table takes into account both the size of the data stored in
+the dynamic table, plus a small allowance for overhead.  The limit on the number
+of blocked streams is only a proxy for the maximum amount of memory required by
+the decoder.  The actual maximum amount of memory will depend on how much memory
+the decoder uses to track each blocked stream.
+
+A decoder can limit the amount of state memory used for the dynamic table by
+setting an appropriate value for the maximum size of the dynamic table. In
+HTTP/3, this is realized by setting an appropriate value for the
+QPACK_MAX_TABLE_CAPACITY parameter. An encoder can limit the amount of state
+memory it uses by signaling lower dynamic table size than the decoder allows

```suggestion
memory it uses by signaling a lower dynamic table size than the decoder allows
```

> +{{maximum-dynamic-table-capacity}} and {{blocked-streams}}). The limit on the
+size of the dynamic table takes into account both the size of the data stored in
+the dynamic table, plus a small allowance for overhead.  The limit on the number
+of blocked streams is only a proxy for the maximum amount of memory required by
+the decoder.  The actual maximum amount of memory will depend on how much memory
+the decoder uses to track each blocked stream.
+
+A decoder can limit the amount of state memory used for the dynamic table by
+setting an appropriate value for the maximum size of the dynamic table. In
+HTTP/3, this is realized by setting an appropriate value for the
+QPACK_MAX_TABLE_CAPACITY parameter. An encoder can limit the amount of state
+memory it uses by signaling lower dynamic table size than the decoder allows
+(see {{eviction}}).  A decoder can limit the amount of state memory used for
+blocked streams by setting an appropriate value for the maximum number of
+blocked streams.  In HTTP/3, this is realized by setting an appropriate value
+for the QPACK_BLOCKED_STREAMS parameter.  An encoder can limit the amount of

nit: QPACK_BLOCKED_STREAMS  vs. SETTINGS_QPACK_BLOCKED_STREAMS

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