diff --git a/der/src/arrayvec.rs b/der/src/arrayvec.rs
index d5cca23da..04fb81344 100644
--- a/der/src/arrayvec.rs
+++ b/der/src/arrayvec.rs
@@ -123,7 +123,7 @@ impl<'a, T> Iterator for Iter<'a, T> {
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let len = self.elements.len() - self.position;
+        let len = self.elements.len().saturating_sub(self.position);
         (len, Some(len))
     }
 }
diff --git a/der/src/asn1/bit_string.rs b/der/src/asn1/bit_string.rs
index b9ce726a3..89f1d31d5 100644
--- a/der/src/asn1/bit_string.rs
+++ b/der/src/asn1/bit_string.rs
@@ -205,6 +205,7 @@ pub struct BitStringIter<'a> {
 impl<'a> Iterator for BitStringIter<'a> {
     type Item = bool;
 
+    #[allow(clippy::integer_arithmetic)]
     fn next(&mut self) -> Option<bool> {
         if self.position >= self.bit_string.bit_len() {
             return None;
@@ -231,6 +232,7 @@ impl<T: flagset::Flags> FixedTag for flagset::FlagSet<T> {
 }
 
 #[cfg(feature = "flagset")]
+#[allow(clippy::integer_arithmetic)]
 impl<'a, T> DecodeValue<'a> for flagset::FlagSet<T>
 where
     T: flagset::Flags,
@@ -256,8 +258,9 @@ where
 }
 
 #[cfg(feature = "flagset")]
+#[allow(clippy::integer_arithmetic)]
 #[inline(always)]
-fn encode<T>(set: &flagset::FlagSet<T>) -> (usize, [u8; 16])
+fn encode_flagset<T>(set: &flagset::FlagSet<T>) -> (usize, [u8; 16])
 where
     T: flagset::Flags,
     u128: From<T::Type>,
@@ -274,6 +277,7 @@ where
 }
 
 #[cfg(feature = "flagset")]
+#[allow(clippy::cast_possible_truncation, clippy::integer_arithmetic)]
 impl<T: flagset::Flags> EncodeValue for flagset::FlagSet<T>
 where
     T::Type: From<bool>,
@@ -281,13 +285,13 @@ where
     u128: From<T::Type>,
 {
     fn value_len(&self) -> Result<Length> {
-        let (lead, buff) = encode(self);
+        let (lead, buff) = encode_flagset(self);
         let buff = &buff[..buff.len() - lead / 8];
         BitString::new((lead % 8) as u8, buff)?.value_len()
     }
 
     fn encode_value(&self, encoder: &mut Encoder<'_>) -> Result<()> {
-        let (lead, buff) = encode(self);
+        let (lead, buff) = encode_flagset(self);
         let buff = &buff[..buff.len() - lead / 8];
         BitString::new((lead % 8) as u8, buff)?.encode_value(encoder)
     }
diff --git a/der/src/asn1/generalized_time.rs b/der/src/asn1/generalized_time.rs
index dce3e402e..2cc7a924b 100644
--- a/der/src/asn1/generalized_time.rs
+++ b/der/src/asn1/generalized_time.rs
@@ -4,8 +4,8 @@ use crate::{
     asn1::Any,
     datetime::{self, DateTime},
     ord::OrdIsValueOrd,
-    ByteSlice, DecodeValue, Decoder, EncodeValue, Encoder, Error, FixedTag, Header, Length, Result,
-    Tag,
+    ByteSlice, DecodeValue, Decoder, EncodeValue, Encoder, Error, ErrorKind, FixedTag, Header,
+    Length, Result, Tag,
 };
 use core::time::Duration;
 
@@ -78,8 +78,12 @@ impl DecodeValue<'_> for GeneralizedTime {
         match *ByteSlice::decode_value(decoder, header)?.as_bytes() {
             // RFC 5280 requires mandatory seconds and Z-normalized time zone
             [y1, y2, y3, y4, mon1, mon2, day1, day2, hour1, hour2, min1, min2, sec1, sec2, b'Z'] => {
-                let year = datetime::decode_decimal(Self::TAG, y1, y2)? as u16 * 100
-                    + datetime::decode_decimal(Self::TAG, y3, y4)? as u16;
+                let year = u16::from(datetime::decode_decimal(Self::TAG, y1, y2)?)
+                    .checked_mul(100)
+                    .and_then(|y| {
+                        y.checked_add(datetime::decode_decimal(Self::TAG, y3, y4).ok()?.into())
+                    })
+                    .ok_or(ErrorKind::DateTime)?;
                 let month = datetime::decode_decimal(Self::TAG, mon1, mon2)?;
                 let day = datetime::decode_decimal(Self::TAG, day1, day2)?;
                 let hour = datetime::decode_decimal(Self::TAG, hour1, hour2)?;
@@ -101,8 +105,8 @@ impl EncodeValue for GeneralizedTime {
     }
 
     fn encode_value(&self, encoder: &mut Encoder<'_>) -> Result<()> {
-        let year_hi = (self.0.year() / 100) as u8;
-        let year_lo = (self.0.year() % 100) as u8;
+        let year_hi = u8::try_from(self.0.year() / 100)?;
+        let year_lo = u8::try_from(self.0.year() % 100)?;
 
         datetime::encode_decimal(encoder, Self::TAG, year_hi)?;
         datetime::encode_decimal(encoder, Self::TAG, year_lo)?;
diff --git a/der/src/asn1/integer/uint.rs b/der/src/asn1/integer/uint.rs
index 4b3812066..a30584abd 100644
--- a/der/src/asn1/integer/uint.rs
+++ b/der/src/asn1/integer/uint.rs
@@ -54,7 +54,7 @@ pub(crate) fn encode_bytes(encoder: &mut Encoder<'_>, bytes: &[u8]) -> Result<()
 #[inline]
 pub(crate) fn encoded_len(bytes: &[u8]) -> Result<Length> {
     let bytes = strip_leading_zeroes(bytes);
-    Length::try_from(bytes.len())? + needs_leading_zero(bytes) as u8
+    Length::try_from(bytes.len())? + u8::from(needs_leading_zero(bytes))
 }
 
 /// Strip the leading zeroes from the given byte slice
diff --git a/der/src/asn1/real.rs b/der/src/asn1/real.rs
index d0fb3c587..36f30e5b7 100644
--- a/der/src/asn1/real.rs
+++ b/der/src/asn1/real.rs
@@ -1,5 +1,12 @@
 //! ASN.1 `REAL` support.
 
+// TODO(tarcieri): checked arithmetic
+#![allow(
+    clippy::cast_lossless,
+    clippy::cast_sign_loss,
+    clippy::integer_arithmetic
+)]
+
 use crate::{
     str_slice::StrSlice, ByteSlice, DecodeValue, Decoder, EncodeValue, Encoder, FixedTag, Header,
     Length, Result, Tag,
@@ -212,6 +219,7 @@ pub(crate) fn mnth_bits_to_u8<const M: usize, const N: usize>(bytes: &[u8]) -> u
 
 /// Decode an f64 as its sign, exponent, and mantissa in u64 and in that order, using bit shifts and masks.
 /// Note: this function **removes** the 1023 bias from the exponent and adds the implicit 1
+#[allow(clippy::cast_possible_truncation)]
 pub(crate) fn decode_f64(f: f64) -> (u64, u64, u64) {
     let bits = f.to_bits();
     let sign = bits >> 63;
diff --git a/der/src/asn1/set_of.rs b/der/src/asn1/set_of.rs
index 80e5e6be4..10f7149cb 100644
--- a/der/src/asn1/set_of.rs
+++ b/der/src/asn1/set_of.rs
@@ -376,9 +376,10 @@ where
 /// This function is used rather than Rust's built-in `[T]::sort_by` in order
 /// to support heapless `no_std` targets as well as to enable bubbling up
 /// sorting errors.
+#[allow(clippy::integer_arithmetic)]
 fn der_sort<T: DerOrd>(slice: &mut [T]) -> Result<()> {
-    for i in 1..=slice.len() {
-        let mut j = i - 1;
+    for i in 0..slice.len() {
+        let mut j = i;
 
         while j > 0 && slice[j - 1].der_cmp(&slice[j])? == Ordering::Greater {
             slice.swap(j - 1, j);
diff --git a/der/src/asn1/utc_time.rs b/der/src/asn1/utc_time.rs
index cbe0f8d86..572537014 100644
--- a/der/src/asn1/utc_time.rs
+++ b/der/src/asn1/utc_time.rs
@@ -4,8 +4,8 @@ use crate::{
     asn1::Any,
     datetime::{self, DateTime},
     ord::OrdIsValueOrd,
-    ByteSlice, DecodeValue, Decoder, EncodeValue, Encoder, Error, FixedTag, Header, Length, Result,
-    Tag,
+    ByteSlice, DecodeValue, Decoder, EncodeValue, Encoder, Error, ErrorKind, FixedTag, Header,
+    Length, Result, Tag,
 };
 use core::time::Duration;
 
@@ -84,7 +84,7 @@ impl DecodeValue<'_> for UtcTime {
         match *ByteSlice::decode_value(decoder, header)?.as_bytes() {
             // RFC 5280 requires mandatory seconds and Z-normalized time zone
             [year1, year2, mon1, mon2, day1, day2, hour1, hour2, min1, min2, sec1, sec2, b'Z'] => {
-                let year = datetime::decode_decimal(Self::TAG, year1, year2)?;
+                let year = u16::from(datetime::decode_decimal(Self::TAG, year1, year2)?);
                 let month = datetime::decode_decimal(Self::TAG, mon1, mon2)?;
                 let day = datetime::decode_decimal(Self::TAG, day1, day2)?;
                 let hour = datetime::decode_decimal(Self::TAG, hour1, hour2)?;
@@ -93,10 +93,11 @@ impl DecodeValue<'_> for UtcTime {
 
                 // RFC 5280 rules for interpreting the year
                 let year = if year >= 50 {
-                    year as u16 + 1900
+                    year.checked_add(1900)
                 } else {
-                    year as u16 + 2000
-                };
+                    year.checked_add(2000)
+                }
+                .ok_or(ErrorKind::DateTime)?;
 
                 DateTime::new(year, month, day, hour, minute, second)
                     .map_err(|_| Self::TAG.value_error())
@@ -114,10 +115,12 @@ impl EncodeValue for UtcTime {
 
     fn encode_value(&self, encoder: &mut Encoder<'_>) -> Result<()> {
         let year = match self.0.year() {
-            y @ 1950..=1999 => y - 1900,
-            y @ 2000..=2049 => y - 2000,
+            y @ 1950..=1999 => y.checked_sub(1900),
+            y @ 2000..=2049 => y.checked_sub(2000),
             _ => return Err(Self::TAG.value_error()),
-        } as u8;
+        }
+        .and_then(|y| u8::try_from(y).ok())
+        .ok_or(ErrorKind::DateTime)?;
 
         datetime::encode_decimal(encoder, Self::TAG, year)?;
         datetime::encode_decimal(encoder, Self::TAG, self.0.month())?;
diff --git a/der/src/datetime.rs b/der/src/datetime.rs
index 19bc64483..6aba89c21 100644
--- a/der/src/datetime.rs
+++ b/der/src/datetime.rs
@@ -57,6 +57,8 @@ pub struct DateTime {
 
 impl DateTime {
     /// Create a new [`DateTime`] from the given UTC time components.
+    // TODO(tarcieri): checked arithmetic
+    #[allow(clippy::integer_arithmetic)]
     pub fn new(year: u16, month: u8, day: u8, hour: u8, minutes: u8, seconds: u8) -> Result<Self> {
         // Basic validation of the components.
         if year < MIN_YEAR
@@ -95,14 +97,14 @@ impl DateTime {
             return Err(ErrorKind::DateTime.into());
         }
 
-        ydays += day as u16 - 1;
+        ydays += u16::from(day) - 1;
 
         if is_leap_year && month > 2 {
             ydays += 1;
         }
 
-        let days = (year - 1970) as u64 * 365 + leap_years as u64 + ydays as u64;
-        let time = seconds as u64 + (minutes as u64 * 60) + (hour as u64 * 3600);
+        let days = u64::from(year - 1970) * 365 + u64::from(leap_years) + u64::from(ydays);
+        let time = u64::from(seconds) + (u64::from(minutes) * 60) + (u64::from(hour) * 3600);
         let unix_duration = Duration::from_secs(time + days * 86400);
 
         if unix_duration > MAX_UNIX_DURATION {
@@ -123,6 +125,8 @@ impl DateTime {
     /// Compute a [`DateTime`] from the given [`Duration`] since the `UNIX_EPOCH`.
     ///
     /// Returns `None` if the value is outside the supported date range.
+    // TODO(tarcieri): checked arithmetic
+    #[allow(clippy::integer_arithmetic)]
     pub fn from_unix_duration(unix_duration: Duration) -> Result<Self> {
         if unix_duration > MAX_UNIX_DURATION {
             return Err(ErrorKind::DateTime.into());
@@ -136,7 +140,7 @@ impl DateTime {
         const DAYS_PER_100Y: i64 = 365 * 100 + 24;
         const DAYS_PER_4Y: i64 = 365 * 4 + 1;
 
-        let days = (secs_since_epoch / 86400) as i64 - LEAPOCH;
+        let days = i64::try_from(secs_since_epoch / 86400)? - LEAPOCH;
         let secs_of_day = secs_since_epoch % 86400;
 
         let mut qc_cycles = days / DAYS_PER_400Y;
@@ -190,12 +194,12 @@ impl DateTime {
         let hour = mins_of_day / 60;
 
         Self::new(
-            year as u16,
+            year.try_into()?,
             mon,
-            mday as u8,
-            hour as u8,
-            minute as u8,
-            second as u8,
+            mday.try_into()?,
+            hour.try_into()?,
+            minute.try_into()?,
+            second.try_into()?,
         )
     }
 
@@ -254,15 +258,19 @@ impl DateTime {
 impl FromStr for DateTime {
     type Err = Error;
 
+    // TODO(tarcieri): checked arithmetic
+    #[allow(clippy::integer_arithmetic)]
     fn from_str(s: &str) -> Result<Self> {
         match *s.as_bytes() {
             [year1, year2, year3, year4, b'-', month1, month2, b'-', day1, day2, b'T', hour1, hour2, b':', min1, min2, b':', sec1, sec2, b'Z'] =>
             {
                 let tag = Tag::GeneralizedTime;
-                let year = decode_decimal(tag, year1, year2).map_err(|_| ErrorKind::DateTime)?
-                    as u16
-                    * 100
-                    + decode_decimal(tag, year3, year4).map_err(|_| ErrorKind::DateTime)? as u16;
+                let year =
+                    u16::from(decode_decimal(tag, year1, year2).map_err(|_| ErrorKind::DateTime)?)
+                        * 100
+                        + u16::from(
+                            decode_decimal(tag, year3, year4).map_err(|_| ErrorKind::DateTime)?,
+                        );
                 let month = decode_decimal(tag, month1, month2).map_err(|_| ErrorKind::DateTime)?;
                 let day = decode_decimal(tag, day1, day2).map_err(|_| ErrorKind::DateTime)?;
                 let hour = decode_decimal(tag, hour1, hour2).map_err(|_| ErrorKind::DateTime)?;
@@ -328,7 +336,7 @@ impl TryFrom<DateTime> for PrimitiveDateTime {
 
     fn try_from(time: DateTime) -> Result<PrimitiveDateTime> {
         let month = (time.month() as u8).try_into()?;
-        let date = time::Date::from_calendar_date(time.year() as i32, month, time.day())?;
+        let date = time::Date::from_calendar_date(i32::from(time.year()), month, time.day())?;
         let time = time::Time::from_hms(time.hour(), time.minutes(), time.seconds())?;
 
         Ok(PrimitiveDateTime::new(date, time))
@@ -342,7 +350,7 @@ impl TryFrom<PrimitiveDateTime> for DateTime {
 
     fn try_from(time: PrimitiveDateTime) -> Result<DateTime> {
         DateTime::new(
-            time.year() as u16,
+            time.year().try_into().map_err(|_| ErrorKind::DateTime)?,
             time.month().into(),
             time.day(),
             time.hour(),
@@ -353,6 +361,8 @@ impl TryFrom<PrimitiveDateTime> for DateTime {
 }
 
 /// Decode 2-digit decimal value
+// TODO(tarcieri): checked arithmetic
+#[allow(clippy::integer_arithmetic)]
 pub(crate) fn decode_decimal(tag: Tag, hi: u8, lo: u8) -> Result<u8> {
     if (b'0'..=b'9').contains(&hi) && (b'0'..=b'9').contains(&lo) {
         Ok((hi - b'0') * 10 + (lo - b'0'))
@@ -362,6 +372,8 @@ pub(crate) fn decode_decimal(tag: Tag, hi: u8, lo: u8) -> Result<u8> {
 }
 
 /// Encode 2-digit decimal value
+// TODO(tarcieri): checked arithmetic
+#[allow(clippy::integer_arithmetic)]
 pub(crate) fn encode_decimal(encoder: &mut Encoder<'_>, tag: Tag, value: u8) -> Result<()> {
     let hi_val = value / 10;
 
diff --git a/der/src/error.rs b/der/src/error.rs
index 551b4cd57..4e0d64aa0 100644
--- a/der/src/error.rs
+++ b/der/src/error.rs
@@ -3,7 +3,7 @@
 pub use core::str::Utf8Error;
 
 use crate::{Length, Tag};
-use core::{convert::Infallible, fmt};
+use core::{convert::Infallible, fmt, num::TryFromIntError};
 
 #[cfg(feature = "oid")]
 use crate::asn1::ObjectIdentifier;
@@ -91,6 +91,15 @@ impl From<Infallible> for Error {
     }
 }
 
+impl From<TryFromIntError> for Error {
+    fn from(_: TryFromIntError) -> Error {
+        Error {
+            kind: ErrorKind::Overflow,
+            position: None,
+        }
+    }
+}
+
 impl From<Utf8Error> for Error {
     fn from(err: Utf8Error) -> Error {
         Error {
diff --git a/der/src/length.rs b/der/src/length.rs
index a27889e29..953bd1209 100644
--- a/der/src/length.rs
+++ b/der/src/length.rs
@@ -145,13 +145,13 @@ impl Sub<Length> for Result<Length> {
 
 impl From<u8> for Length {
     fn from(len: u8) -> Length {
-        Length(len as u32)
+        Length(len.into())
     }
 }
 
 impl From<u16> for Length {
     fn from(len: u16) -> Length {
-        Length(len as u32)
+        Length(len.into())
     }
 }
 
@@ -202,9 +202,10 @@ impl Decode<'_> for Length {
                 let nbytes = tag.checked_sub(0x80).ok_or(ErrorKind::Overlength)? as usize;
                 debug_assert!(nbytes <= 4);
 
-                let mut decoded_len = 0;
+                let mut decoded_len = 0u32;
                 for _ in 0..nbytes {
-                    decoded_len = (decoded_len << 8) | decoder.byte()? as u32;
+                    decoded_len = decoded_len.checked_shl(8).ok_or(ErrorKind::Overflow)?
+                        | u32::from(decoder.byte()?);
                 }
 
                 let length = Length::try_from(decoded_len)?;
@@ -237,6 +238,7 @@ impl Encode for Length {
         }
     }
 
+    #[allow(clippy::cast_possible_truncation)]
     fn encode(&self, encoder: &mut Encoder<'_>) -> Result<()> {
         if let Some(tag_byte) = self.initial_octet() {
             encoder.byte(tag_byte)?;
diff --git a/der/src/lib.rs b/der/src/lib.rs
index e8dacd7c6..825d307d4 100644
--- a/der/src/lib.rs
+++ b/der/src/lib.rs
@@ -8,6 +8,14 @@
 )]
 #![forbid(unsafe_code)]
 #![warn(
+    clippy::cast_lossless,
+    clippy::cast_possible_truncation,
+    clippy::cast_possible_wrap,
+    clippy::cast_precision_loss,
+    clippy::cast_sign_loss,
+    clippy::checked_conversions,
+    clippy::implicit_saturating_sub,
+    clippy::integer_arithmetic,
     clippy::panic,
     clippy::panic_in_result_fn,
     clippy::unwrap_used,
diff --git a/der/src/tag.rs b/der/src/tag.rs
index 7597da6b7..3aa102c60 100644
--- a/der/src/tag.rs
+++ b/der/src/tag.rs
@@ -353,7 +353,8 @@ impl fmt::Display for Tag {
             } => write!(
                 f,
                 "APPLICATION [{}] ({})",
-                number, FIELD_TYPE[constructed as usize]
+                number,
+                FIELD_TYPE[usize::from(constructed)]
             ),
             Tag::ContextSpecific {
                 constructed,
@@ -361,7 +362,8 @@ impl fmt::Display for Tag {
             } => write!(
                 f,
                 "CONTEXT-SPECIFIC [{}] ({})",
-                number, FIELD_TYPE[constructed as usize]
+                number,
+                FIELD_TYPE[usize::from(constructed)]
             ),
             Tag::Private {
                 constructed,
@@ -369,7 +371,8 @@ impl fmt::Display for Tag {
             } => write!(
                 f,
                 "PRIVATE [{}] ({})",
-                number, FIELD_TYPE[constructed as usize]
+                number,
+                FIELD_TYPE[usize::from(constructed)]
             ),
         }
     }
diff --git a/der/src/tag/class.rs b/der/src/tag/class.rs
index f421bbe4a..8a3e2ed10 100644
--- a/der/src/tag/class.rs
+++ b/der/src/tag/class.rs
@@ -32,8 +32,9 @@ pub enum Class {
 
 impl Class {
     /// Compute the identifier octet for a tag number of this class.
+    #[allow(clippy::integer_arithmetic)]
     pub(super) fn octet(self, constructed: bool, number: TagNumber) -> u8 {
-        self as u8 | number.value() | (constructed as u8 * CONSTRUCTED_FLAG)
+        self as u8 | number.value() | (u8::from(constructed) * CONSTRUCTED_FLAG)
     }
 }